Signal intercepting circuit

ABSTRACT

The muting circuit of the present invention comprises a first and second differential amplifier. The first differential amplifier comprises first and second transistors. The base electrode of the first transistor receives an input signal. The second differential amplifier comprises third and fourth transistors. The collector electrode of the third transistor is connected to the collector electrode of the second transistor, while the collector electrode of the fourth transistor is connected to the collector electrode of the first transistor. A switching circuit, receiving an intercepting signal, is coupled to the first and second differential amplifier. In the absence of an intercepting signal, the switching circuit causes the conduction of the first differential amplifier, and while cutting off the second differential amplifier. Upon receiving an intercepting signal, the switching circuit cuts off the first differential amplifier while it causes the conduction of second differential amplifier. Furthermore, the switching circuit causes the simultaneous conduction of the first and second differential amplifiers during a rising time and falling time of the intercepting signal. A D.C. fluctuation in the output voltage is produced during the time one amplifier is being activated while the other is being cut off. The simultaneous operation of both differential circuits prevents D.C. fluctuations in input voltage supplied to an output transistor. Consequently, the D.C. output potential of an output amplifier does not substantially fluctuate irrespective of the intercepting operation.

BACKGROUND OF THE INVENTION

This invention relates to a signal intercepting circuit and particularly, to a muting circuit for intercepting an audio signal supplied to a loudspeaker in an audio amplifying circuit in a radio, such as a stereophonic receiving apparatus, or a television receiver.

In an audio amplifying circuit, a muting circuit is used to temporarily prevent an audio signal from being supplied to the loudspeaker when a power supply switch is turned on, or when the band selecting or STEREO MONO selecting switches is changed.

A muting circuit of the prior art is shown in FIG. 1. An audio signal is supplied to an input terminal 11. Terminal 11 is connected, through a resistor R₁₁, to the base electrode of a transistor Q₁₁. The base electrode of transistor Q₁₁ is also grounded through a resistor R₁₃. Its collector electrode is grounded and its emitter electrode is connected to the emitter electrode of a transistor Q₁₂. The collector electrode of transistor Q₁₂ is connected to an output terminal 14 through an amplifier 12. The base electrode of transistor Q₁₂ is connected, through a negative feedback resistor R₁₂, to output terminal 14 and to ground through serially connected condenser C₁₁ and resistor R₁₄. Output terminal 14 is connected to a loudspeaker (not shown). Further, the collector electrode of a muting transistor Q₁₃ is connected to the base electrode of transistor Q₁₁, while its emitter electrode is grounded. A muting pulse is supplied to the base electrode of transistor Q₁₃ in response to, for example, the turning on of a power supply switch (not shown).

The operation of the muting circuit is as follows: when a positive muting pulse is supplied to the base electrode of transistor Q₁₃, transistor Q₁₃ is turned on. Accordingly, the audio signal is bypassed to the ground through transistor Q₁₃, rather than being supplied to the base electrode of transistor Q₁₁.

At the transient time transistor Q₁₁ is turned on, its base potential, however, drops from I_(B) R_(B) to zero volts. As a result, a large AC pulsed waveform is produced at output terminal 14 which is thereby supplied to the loudspeaker. This pulsed AC waveform causes the loudspeaker to produce a "pop" sound. I_(B) is the base current of transistor Q₁₁ and R_(B) is the resistance of resistor R₁₃.

In order to prevent the base potential of transistor Q₁₁ from dropping, a condenser (not shown) must be connected between the collector electrode of transistor Q₁₃ and the terminal of resistor R₁₃ which is connected to the base electrode of transistor Q₁₁. Since the capacitance of such a condenser must be considerably large, the resulting muting circuit has the disadvantage that it cannot be fabricated into a semiconductor integrated circuit.

SUMMARY OF THE INVENTION

Accordingly, an object of this invention is to provide an improved signal intercepting circuit which does not produce an undesirable AC component during the functioning of the intercepting operation.

Another object of this invention is to provide an intercepting circuit which can be readily fabricated into a semiconductor integrated circuit.

The muting circuit of the present invention comprises a first and second differential amplifier. The first differential amplifier comprises first and second transistors. The base electrode of the first transistor receives an input signal. The second differential amplifier comprises third and fourth transistors. The collector electrode of the third transistor is connected to the collector electrode of the second transistor, while the collector electrode of the fourth transistor is connected to the collector electrode of the first transistor. A switching circuit, receiving an intercepting signal, is coupled to the first and second differential amplifier. In the absence of an intercepting signal, the switching circuit causes the conduction of the first differential amplifier, and while cutting off the second differential ampifier. Upon receiving an intercepting signal, the switching circuit cuts off the first differential amplifier while it causes the conduction of the second differential amplifier. Furthermore, the switching circuit causes the simultaneous conduction of the first and second differential amplifiers during a rising time and falling time of the intercepting signal. A D.C. fluctuation in the output voltage is produced during the time one amplifier is being activated while the other is being cut off. The simultaneous operation of both differential circuits prevents D.C. fluctuations in input voltage supplied to an output transistor. Consequently, the D.C. output potential of an output amplifier does not substantially fluctuate irrespective of the intercepting operation.

The objects and advantages of the present invention will become apparent to persons skilled in the art from a study of the following description of the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic circuit diagram of a muting circuit of the prior art.

FIG. 2 is a circuit diagram of a muting circuit according to the present invention.

FIG. 3 is a schematic circuit diagram of another embodiment according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention will be described in detail with reference to the accompanying drawings. Throughout the drawings, like reference numerals will be used to designate like or equivalent portions, for simplicity in explanation.

Referring now to FIG. 2, an audio signal S is supplied to the base electrode of a transistor Q₂₁. The emitter electrode of transistor Q₂₁ is connected to the emitter electrode of a transistor Q₂₂. Transistors Q₂₁ and Q₂₂ constitute a first differential amplifier. The base electrode of a transistor Q₂₃ is connected to the base electrode of transistor Q₂₂, while its emitter electrode is connected to the emitter electrode of a transistor Q₂₄. The collector electrode of transistor Q₂₃ is connected to the collector electrode of transistor Q₂₂. The collector electrode of transistor Q₂₄ is connected to the collector electrode of transistor Q₂₁. Transistors Q₂₃ and Q₂₄ constitute a second differential amplifier which form a doubly balanced differential amplifier 21 together with transistors Q₂₁ and Q₂₂.

The collector electrode of a transistor Q₂₅ is connected to the emitter electrode of transistor Q₂₁, while its emitter electrode is grounded through a resistor R₂₁. The collector electrode of a transistor Q₂₆ is connected to the emitter electrode of transistor Q₂₃, while its emitter electrode is grounded through a resistor R₂₂. Transistors Q₂₅ and Q₂₆ constitute a constant current source circuit 22 for transistors Q₂₁, Q₂₂ and Q₂₃, Q₂₄.

Resistor R₃₀ and diodes D₂₃, D₂₄ are serially connected between a power supply line 20 and ground. The connection point of resistor R₃₀ and diode D₂₃ is connected, through a resistor R₃₁, to the base electrode of transistor Q₂₁. The connection point of diodes D₂₃, D₂₄ is connected to the base electrodes of transistors Q₂₅, Q₂₆. Resistors R₃₀, R₃₁ and diodes D₂₃, D₂₄ constitute a bias circuit 27 for transistors Q₂₁, Q₂₅ and Q₂₆. Resistor R₃₂ and diodes D₂₅, D₂₆ are serially connected between power supply line 20 and ground. The connection point of resistor R₃₂ and diode D₂₅ is connected, through a resistor R₃₃, to the base electrode of transistor Q₂₄. Resistors R₃₂, R₃₃ and diodes D₂₅, D₂₆ constitute a bias circuit 38 for transistor Q₂₄.

The collector electrode of a transistor Q₂₇ is connected to the collector electrode of transistor Q₂₁, while its emitter electrode is connected, through a resistor R₂₃, to power supply line 20. The collector electrode of a transistor Q₂₈ is connected to the collector electrode of transistor Q₂₃, while its emitter electrode is connected, through a resistor R₂₄, to power supply line 20. The base electrode of transistor Q₂₈ is connected to its collector electrode and to the base electrode of transistor Q₂₇. Transistors Q₂₇ and Q₂₈ constitute a current mirror circuit which functions as a load circuit 23 for doubly balanced differential amplifier 21.

The collector electrode of a transistor Q₂₉ is connected to the emitter electrode of transistor Q₂₁, while its emitter electrode is connected, through a resistor R₂₅, to power supply line 20. The collector electrode of a transistor Q₃₀ is connected to the emitter electrode of transistor Q₂₃, while its emitter electrode is connected to the emitter electrode of transistor Q₂₉. Transistors Q₂₉, Q₃₀ and resistor R₂₅ constitute a current supply circuit 24.

The base electrode of transistor Q₃₀ is connected to power supply line 20 through a transistor Q₃₁, functioning as a diode, and to ground through serially connected resistor R₂₆ and diode D₂₂. The connection point of transistor Q₃₁ and resistor R₂₆ is connected, through a diode D₂₁, to the base electrode of transistor Q₂₉. Transistor Q₃₁, resistor R₂₆ and diode D₂₂ constitute a bias circuit for transistor Q₃₀. Diode D₂₁ prevents the base potential of transistor Q₂₉ from dropping uder a predeterminate value.

The base electrode of transistor Q₂₉ is connected, through a resistor R₂₇, to power supply line 20 and to the collector electrode of transistor Q₃₂ through a resistor R₂₈. The emitter electrode of transistor Q₃₂ is grounded through a resistor R₂₉. Transistor Q₃₂ and resistors R₂₇, R₂₈ and R₂₉ constitute an input circuit 26 for receiving a muting pulse P. The muting pulse P is supplied to the base electrode of transistor Q₃₂. Circuits 24, 25 and 26 constitute a switching circuit which control the operation of the two differential amplifiers (i.e., Q₂₁, Q₂₂ and Q₂₃, Q₂₄) of circuit 21.

The collector electrode of transistor Q₂₁ is also connected to the base electrode of a transistor Q₃₃. The collector electrode of transistor Q₃₃ is connected to power suply line 20, while its emitter electrode is connected, through an amplifier 29, to an output terminal 30. Output terminal 30 is connected to the base electrode of transistors Q₂₂ and Q₂₃ through a negative feedback resistor R₃₄ ; the base electrodes of transistors Q₂₂ and Q₂₃ are also grounded through serially connected condenser C₂₁ and resistor R₃₅.

The operation of the muting circuit will now be described. As shown, it is assumed that I₁, I₂ and I₃ are the values of the constant currents flowing through transistors Q₂₅, Q₂₆ and resistor R₂₅, respectively. Further, the circuit is designed so that the relationship among I₁, I₂ and I₃ is as follows:

    I.sub.1 <I.sub.3 <I.sub.1 +I.sub.2                         (1)

    I.sub.2 <I.sub.3 <I.sub.1 +I.sub.2                         (2)

During normal operations (i.e., a muting pulse is not supplied to the base electrode of transistor Q₃₂), transistor Q₃₀ conducts and transistor Q₂₉ is cut off. Since transistor Q₂₉ is cut off, constant current I₃ flowing through resistor R₂₅ flows through transistor Q₃₀. Except for a small portion of current I₃ flowing from the emitter to the base electrode of transistor Q₃₀, most of the current (i.e., I₃ ') flows through the collector electrode of Q₃₀ and to the interconnection of the emitter electrodes of transistors Q₂₃, Q₂₄. Transistor Q₃₀ is selected so that the amount of its emitter-base current flow, results in current I₃ ' being equal to constant current I₂. Since I₃ ' equals I₂, no additional current is needed from the emitter electrodes of transistors Q₂₃, Q₂₄ to supply current to node 2. Consequently, the current supplied from transistors Q₂₃, Q₂₄ to node 2 is zero; as a result, transistors Q.sub. 23, Q₂₄ are forced to cut off. Transistors Q₂₁, Q₂₂, however, continue to conduct. As a result, audio signal S is amplified by differential transistors Q₂₁, Q₂₂, transistor Q₃₃ and amplifier 29, and then supplied to output terminal 30.

The muting operation will now be described upon application of a muting pulse P to the base electrode of transistor Q₃₂. This pulse will generally be supplied when the power supply switch is turned on, or when the band selecting or STEREO MONO selecting switches is changed. In response to muting pulse P, transistor Q₃₂ is activated thereby causing conduction of transistor Q₂₉, while transistor Q₃₀ is cut off. Since transistor Q₃₀ is cut off, constant current I₃ flows through transistor Q₂₉. Except for a small portion of current flowing from the emitter to the base electrode of transistor Q₂₉, most of the current (i.e., I₃ ') flows through the collector electrode of Q₂₉ and to the interconnection of the emitter electrodes of transistors Q₂₁, Q₂₂. Transistor Q₂₉ is selected so that the amount of its emitter-base current flow, results in current I₃ ' being equal to I₁. Since I₃ ' equals I₁, no additional current is needed from the emitter electrodes of transistors Q.sub. 21, Q₂₂ to supply current to node 1. Consequently, the current supplied from transistors Q₂₁, Q₂₂ to node 1 is zero; as a result, transistors Q₂₁, Q₂₂ are forced to cut off. Accordingly, the audio signal S is not supplied to output terminal 30, and the muting operation is achieved. Although transistors Q₂₁, Q₂₂ are cut off, transistors Q₂₃, Q₂₄ continue to conduct and supply a constant D.C. voltage to the input terminal of transistor Q₃₃. Consequently, the D.C. potential supplied to output terminal 30 does not substantially fluctuate despite the presence or absence of muting.

Further, when transistor Q₃₂ receives either the rising or falling edge of muting pulse P, both transistors Q₂₉, Q₃₀ conduct resulting in constant current I₃ flowing through transistors Q₂₉, Q₃₀. In view of the relation of I₃ <I₁ +I₂, additional current is needed from the emitter electrodes of transistors Q₂₁, Q₂₂ to supply current to node 1 and additional current is needed from the emitter electrodes of transistor Q₂₃, Q₂₄ to supply current to node 2. Consequently, transistors Q₂₁ to Q₂₄ conduct to supply the necessary current. The simultaneous operation of both differential amplifier prevents D.C. fluctuations in the input voltage supplied to output transistor Q₃₃ during the rising and falling time of pulse P. Consequently, the D.C. potential supplied to output terminal 30 does not substantially fluctuate irrespective of the intercepting operation.

Furthermore, the presence of diode D₂₁ between the base electrode of transistor Q₂₉ and bias circuit 25, prevents the base potential of transistor Q₂₉ from dropping below a predetermined value despite conduction of transistor Q₃₂. As a result, an undesirable current is prevented from flowing from the emitter electrode of transistor Q₂₁ into the base electrode of transistor Q₂₉ via the collector electrode of transistor Q₂₉.

The muting circuit of the present invention does not employ a condenser as used in the prior art. Accordingly, it can be easily fabricated into a semiconductor integrated circuit. Further, since the number of transistors connected between power supply line 20 and ground is small, the power supply requirements are small.

The muting circuit shown in FIG. 1 may be modified as shown by the circuit arrangement of FIG. 3. As shown, rather then emplying transistors Q₂₅, Q₂₆ (see FIG. 2), constant current source circuit 22 can consist of either resistors R₄₁, R₄₂ having a high resistance, or constant current sources I₂₁, I₂₂. Rather than employing current mirror circuit Q₂₇, Q₂₈ (FIG. 2), load circuit 23 consists of resistors R₄₃, R₄₃ ' respectively connected between power supply line 20 and the collector electrodes of transistor Q₂₁, Q₂₃. Further, rather than employing bias circuits 27, 28 (FIG. 2), reference voltage sources E₂₁ and E₂₂ are respectively employed.

Although illustrative embodiments of the invention have been described in detail with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments and that various changes and modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the invention. 

What is claimed is:
 1. A signal intercepting circuit for receiving an intercepting signal having a rising edge and a falling edge, said rising edge occurring during a rising time period and said falling edge occurring during a falling time period, comprising:a first differential amplifier comprising first and second transistors, the base electrode of said first transistor receiving an input signal; a second differential amplifier comprising third and fourth transistors, the collector electrode of said third transistor being connected to the collector electrode of said second transistor, and the collector electrode of said fourth transistor being connected to the collector electrode of said first transistor; switching means coupled to said first and second differential amplifier and responsive to said intercepting signal, for switching off said first differential amplifier and switching on said second differential amplifier, said switching means comprising sensing means for sensing the absence of said intercepting signal and switching on said first differential amplifier and switching off said second differential amplifier, said switching means further comprising means for simultaneously switching on said first and second differential amplifiers during said rising time period and said falling time period; and, an output terminal coupled to said first differential amplifier.
 2. The signal intercepting circuit of claim 1 further comprising:a first constant current source circuit coupled to the emitter electrodes of said first and second transistors; and, a second constant current source circuit coupled to the emitter electrodes of said third and fourth transistors.
 3. The signal intercepting circuit of claim 2 wherein said switching means comprises current supply means coupled to said sensing means for supplying a constant current to the emitter electrodes of said third and fourth transistors during the absence of said intercepting signal, to the emitter electrodes of said first and second transistors during the presence of said intercepting signal, and, to the emitter electrodes of said first, second, third and fourth transistors during said rising time period and said falling time period of said intercepting signal. 